Friction stir welding is a welding method invented by TWI (The Welding Institute) in the United Kingdom in 1991, in which members to be welded are stirred (to induce plastic flow) after the deformation resistance of members to be welded is reduced by frictional heat, there achieving welding. In friction stir welding, welding is performed in a solid phase, unlike general fusion welding, in which a junction is fused. Consequently, it is known that the structure of the junction is refined, thereby exhibiting excellent mechanical properties.
To date, the practical application of friction stir welding has become greatly prevalent in the industrial world, and various kinds of research on further application to new fields have been enthusiastically carried out. In particular, a method of easily welding members to be welded having a large plate thickness, which are difficult to weld, or easily welding members to be welded made of different metal materials thanks to the characteristics of friction stir welding is required.
Welding conditions for friction stir welding generally includes the load of a rotary tool, the rotational speed of the rotary tool, the movement speed of the rotary tool, and the forward angle of the rotary tool, the parameters of which are adjusted to achieve desired welding. The most important factor in examining the welding conditions for friction stir welding is heat input to the members to be welded. In order to enable plastic flow of a junction due to stirring, it is necessary to provide a quantity of heat sufficient to reduce the deformation resistance of the members to be welded. Meanwhile, excessive heat input unnecessarily softens the members to be welded, unnecessary burrs are generated by gouging the junction, or the mechanical properties of the junction are reduced due to the slow cooling rate.
In addition, it is understood that it is also difficult to achieve welding of members to be welded having a large plate thickness or welding of members to be welded made of different materials due to heat input during welding. Particularly, in order to achieve welding of members to be welded having a large plate thickness, it is necessary to generate uniform plastic flow over the entire plate thickness, and uniform heat input is required from the welding surface against which the rotary tool is pushed to the rear surface. Moreover, in order to achieve welding of members to be welded made of different materials, it is necessary to mix two kinds of members to be welded as uniformly as possible due to plastic flow, and the deformation resistances of the two kinds of members to be welded must be reduced so as to be almost equal to each other by controlling heat input during welding.